1. Field of the Invention
The present invention relates to a fan system and brake control circuit thereof, particularly to a brake control circuit of a fan system including a buffer circuit to gradually stop the operation of the fan system.
2. Description of the Related Art
Recently, it is general in relative fields to complete electronic products by a combination of micro-controller unit (MCU) and a drive circuit with electronic power switches. However, in operation of the electronic product, a great amount of heat is generated due to high frequency control of the MCU and electronic power switches, and thus cooling efficiency of the electronic product becomes an important issue. In the present day, a conventional way to achieve a cooling purpose is to dissipate heat generated by the electronic product with a fan system. Besides, a stop control circuit is usually applied to the fan system to provide some functions in speed control and operation security, so as to rapidly stop rotation of a fan wheel of the fan system after operating power of the fan system is off.
Referring to FIG. 1, a conventional fan system includes a bridge driver 91, a control unit 92, a reservoir capacitor 93, a brake circuit 94, and a motor coil 95. In order to provide the above-mentioned elements with required power, a voltage source VCC is connected to the bridge driver 91, control unit 92, reservoir capacitor 93, and brake circuit 94. The bridge driver 91 includes two upper bridge power switches M91, M94 preferably being P-type transistors such as P-channel MOSFETs and two lower bridge power switches M92, M93 preferably being N-type transistors such as N-channel MOSFETs. The brake circuit 94 includes two power switches M95, M96 respectively connecting with the two lower bridge power switches M92, M93 of the bridge driver 91. The control unit 92 includes four control output ports separately connecting with the two power switches M95, M96 and another two power switches M97, M98, which respectively connect with and control the two upper bridge power switches M91, M94. The reservoir capacitor 93 serially links the earth and a joint between the voltage source VCC and the bridge driver 91. The motor coil 95 includes two ends, with one of the two ends connecting with a joint between the upper bridge power switch M91 and the lower bridge power switch M93, and with the other end connecting with a joint between the upper bridge power switch M94 and the lower bridge power switch M92.
When the voltage source VCC normally provides the fan system with power, the control unit 92 sends out a set of control signals to operate the bridge driver 91 and the brake circuit 94 for a current to alternatively flow from the voltage source VCC to the ground in two paths, with one of the paths going through the upper bridge power switch M91, motor coil 95, and lower bridge power switch M92 in sequence while the other one of them goes through the upper bridge power switch M94, motor coil 95, and lower bridge power switch M93 in sequence. Therefore, with said two paths, the motor coil 95 is provided with an alternating current to generate alternate magnetic fields. Furthermore, in this situation, the voltage source VCC continuously charges the reservoir capacitor 93 till the reservoir capacitor 93 stores power energy of a predetermined level.
Alternatively, when the voltage source VCC is turned off, the control unit 92 does not generate the control signals for the bridge driver 91 and brake circuit 94 any more, and therefore the two switches M95, M96 are off. Besides, in this situation, with the electrical power previously stored in the reservoir capacitor 93 and the connection between the reservoir capacitor 93 and two control ports of the two lower bridge power switches M92, M93, said two control ports are switched on by the pre-stored electrical power. Accordingly, the two ends of the motor coil 95 are forced to be at the same voltage level, and therefore the fan system is actuated to stop rapidly.
However, there are still some drawbacks of this conventional fan system shown as the following. Due to an immediate change in voltage across the motor coil 95 at the moment when the voltage source VCC is turned off and voltage levels of the ends are made equal suddenly and forcibly, a current passing through the motor coil 95 is suddenly dropped to zero and induces an electromotive force resulting in electromagnetic interference. In the meantime, with the two ends of the motor coil 95 being grounded through the lower bridge power switches M92, M93, the induced electromotive force is immediately discharged and causes a great magnetic stress on the motor coil 95. Therefore, vibration and noise generated by the fan system are increased, which further leads to low operational stability of peripheral circuits and short lifetime of the fan system. Hence, there is a need of improvement in the conventional fan system and brake control circuit thereof.